On 03/07/12 11.15, Dr. H. Nikolaus Schaller wrote:
...
Now comes the crucial question: how can this be improved?
The specialists of the production company have spent whole weekends
to do more experiments but did not yet find the right trick. It is not that
they do not want to solve the problem (they see it as a challenge) they
simply have not yet found the key.
...
Unless one of the readers on these list knows some real specialist
for such PoP soldering of BGAs who is willing and capable to
consult (even if it costs money). So if you know someone, please
let me know.
...
Hi Nikolaus
I have no experience with BGA soldering, but here are some (untested)
suggestions. I have no idea what is possible. Maybe it might add to a
solution?:
* Use an heat infrared camera to "see" during soldering (and passive
cooling) how the BGA-chip temperature is distributed during the process
- if possible - even of the PCB underside.
* If BGA-chip bulges it maybe that it is too "cool" at the periphery?
Can the periphery be heated more? Or be heat-isolated more during
passive cooling - or cool the BGA-center more than the periphery? Maybe
the PCB just opposite the BGA should have the opposite treatment during
the cooling process?
* Make hollow BGA PCB-islands where to much or to little solder can flow
through? The melted solder adhersion should keep the solder where it
belongs?
* Maybe this paper has the answer?:
BGA rework
A comparitive study of selective solder paste deposition for area array
packages.
Ray Cirimele BEST, Inc. Rolling Meadows, Illinois rcirimele@solder....:
http://www.solder.net/stencilquik/papers/SMT_PANPAC_Paper_Rev032704.pdf
Quote: "...
There has been much controversy over whether BGA solder joint
reliability is better when processed with solder paste versus paste
flux. Some studies1,2 indicate that the solder joint shape may have a
greater impact on the solder joint reliability than the volume.
...
Figure 1. An open connection caused from poor coplanarity and the use of
flux instead of paste.
...
Many things can affect the coplanarity. If the BGA has solder balls that
are not coplanar it can result in a "open connection" (a ball that has
not contacted and wetted the land while surrounding balls have made
contact). Variations in surface finish height, warp of the board or
area, and warp or bow of the device itself can create coplanarity
problems. The use of solder paste can help to overcome variations in
solder ball height. The use of solder paste may also improve wetting as
a result of a greater area of intimate contact between the ball and the
paste and the land and the paste. Solder paste may provide better "tack"
than flux alone. On newer technology PCBs, the utilization of drilled
(usually laser) micro-vias in the center of the BGA land requires the
use of solder paste to reduce voiding and to prevent loss of the solder
ball volume as a result of filling the via.
...
Although there are many ways to replace the Area Array devices using
flux, solder paste, or solder bumps, only the four most common methods
of solder paste deposition will be discussed.
...
Stand Off
There are some BGA devices with eutectic solder balls that tend to
exhibit a great deal of ball collapse after reflow. This can be caused
from design (land size) or BGA weight. Many times this excessive
collapse can result in solder shorts (usually in the corners).
Generally, greater stand-off height in conjunction with proper solder
volumes and shape can provide better solder joint reliability. The semi-
permanent stencil will prevent excessive collapse of the solder balls
and can be used to provide a minimum stand-off height.
Solder Shorts
Solder shorts can occur on BGA devices for a number of reasons. Most of
these reasons have to do with the solder paste deposition or excessive
collapse of the solder balls causing an increase in the ball diameter
and a reduction in the ball spacing. One clear advantage that the semi-
permanent stencil has over the other stencils is that it acts as a
physical barrier to prevent solder shorts from occurring. The
non-wettable polyimide film holds the solder paste in the stencil
apertures, and once the solder becomes molten it coalesces and cannot
migrate over the stencil.
...
Solder Volume
Based on previous works done on the relationship between BGA solder
joint quantity and solder joint strength and reliability, greater solder
quantities generally result in greater joint strength. Some studies
indicate that solder joint shape may have as much to do with joint
strength as the solder volume. >>Greater stand-off heights with a fillet
shaped like an hour glass may provide better reliability than a
traditional rounded collapsed ball shape1,2.<<
...
CONCLUSION
Although stencils have been used in the electronics industry for many
years, the use has been plagued by the impact of the removal of the
stencil from the substrate. Through the use of compatible materials it
has become possible to omit the stencil removal process step and thereby
remove many of the potential opportunities for error. The improvements
to the process include:
* Ease of use
* No paste release problems
* No smearing during removal
* Prevention of solder shorts
* Repairs damaged solder resist
The fact that by leaving the stencil in place benefits the process of
BGA replacement by opening the process window is an added bonus.
..."
Search:
http://www.google.dk/search?q=university+solder+bga+joint+study
-
AN 353: SMT Board Assembly Process Recommendations:
http://www.altera.com/literature/an/an353.pdf
Quote: "...
One important factor that you must consider in designing stencils is
that lead-free pastes have higher surface tension and do not wet or
spread on the surface of pads as easily as eutectic solder pastes. This
higher surface tension can lead to exposed pad finish material after
reflow soldering. You can rectify this problem by modifying the stencil
aperture designs to increase the paste coverage on the pads.
...
To ensure that all components are heated to temperatures above the
minimum reflow temperatures and that smaller components do not exceed
the maximum temperature limits, you must perform reflow profiling by
attaching calibrated thermocouples embedded in the spheres of the larger
BGA parts as well as other critical locations on the boards (2). Because
the components are subjected to higher reflow temperatures, select the
appropriate moisture sensitivity level (MSL) for the components and
component handling. You must strictly follow the storage recommendations.
Although nitrogen is not required, Altera recommends including nitrogen
in the reflow process as its presence helps achieve better wettability
and widen the process window. Nitrogen is especially beneficial when
temperature differential across the board is large. Additionally,
nitrogen improves the appearance of solder joints by inhibiting the
effects of oxidation.
...
To ensure that all packages are successfully and reliably assembled, the
reflow profiles studied and recommended by Altera are based on the
JEDEC/IPC standard J-STD-020 revision D.1 (3).
...
To reduce thermal stress on boards and components, you must control the
peak temperatures below the recommended maximums (Table 4) and minimize
the temperature gradients across the board. High temperatures can put
significant stress on plated through-holes and barrels, which can lead
to cracking. High first-pass temperatures on double-sided assemblies
increase the amount of second-side oxidation, which can cause
solderability problems on the second pass.
Altera has worked extensively with leading EMS companies and has
successfully demonstrated that the Pb-free parts can be soldered in air
atmosphere (5).
However, for high-density, two-sided assemblies, you can alleviate the
problems related to a narrow process window by selecting modern reflow
ovens with forced convection and more heating zones with tighter process
controls on reflow parameters. Reflow ovens equipped with nitrogen
reflow atmosphere have shown to improve wettability at lower peak
temperatures and reduce temperature gradients across the board and have
proven beneficial for double-sided assemblies (6).
..."
PCB Layout Recommendations for BGA Packages
http://www.latticesemi.com/lit/docs/technotes/tn1074.pdf
http://www.eetasia.com/STATIC/PDF/200906/EEOL_2009JUN16_EDA_AN_01.pdf?SOURCES=DOWNLOAD
Application Note AN-1028
Recommended Design, Integration and Rework Guidelines for International
Rectifier's BGA and LGA Packages
by Kevin Hu, International Rectifier
http://www.irf.com/technical-info/appnotes/an-1028.pdf
Search:
http://www.google.dk/search?q=solder+bga+recommendation
-
http://www.google.dk/search?q=site%3Axilinx.com+bga+solder
-
http://www.google.dk/search?q=coplanarity+bga+warp
NASA guidelines for ball grid array selection and use:
http://nepp.nasa.gov/docuploads/87ED1BD1-16C1-4F5E-A1227047B50C6CBE/BGA%20Guidelines%20090706.doc
Quote: "...
9.6. Coplanarity
Generally, the coplanarity of the BGA is very small since there are no
leads to bend and the plane of the solder bumps is flat. The warp of
the printed circuit board will generally dictate the coplanarity of the
BGA assembly. It is recommended that the allowable warp of a PCB for
BGA use be held to less than 1%; 1% warp could be greater than 0.010
inch under a 35 mm square BGA. An allowable PCB warp of 0.5% max is
recommended.
...
10.3 Part Moisture Control
Prior to population of flight printed circuit assemblies with PBGA's or
other plastic IC's, it must be verified that controls for moisture have
been followed in accordance with IPC/JEDEC J-STD-033A. Bake-out of
PBGA's at 125oC for 4 hours is recommended followed by dry storage until
use. The time between removal from storage and reflow is dependent upon
the parts sensitivity factor per J-STD-033A and must be strictly
followed. PCB's should be baked at 120oC for 3.5 hours.
...
Appendix A
Process Evaluation Guideline
..."
Implementing High Temperature Coplanarity Requirements for Components
and PWBs:
http://thor.inemi.org/webdownload/projects/ba/Board_Coplanarity/Board_Coplanarity_Background.pdf
Quote: "...
* Its clear that numbers don't add up properly, yet packages surface
mount anyway .Most components and boards do not approach the maximum
allowable
* However the case above is possible according to the specs, but not
probable
..."
CSN33: Micron BGA Manufacturer's User Guide:
http://www.micron.com/~/media/Documents/Products/Customer%20Service%20Note/6882csn33_bga_user_guide.pdf
-
br,
Glenn
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